Large quantities of natural gas are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, a significant amount of natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane, the main component of natural gas, as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into the higher hydrocarbons.
Current industrial use of methane or natural gas as a chemical feedstock proceeds by the initial conversion of the feedstock to carbon monoxide and hydrogen by either steam reforming (the most widespread process), dry reforming, autothermal reforming, or catalytic partial oxidation. Examples of these processes are disclosed in GUNARDSON, HAROLD, Industrial Gases in Petrochemical Processing 41-80 (1998). An example of catalytic partial oxidation is shown in U.S. Published patent application Ser. No. 20020013227 to Dindi et al., both incorporated herein by reference for all purposes. Steam reforming, dry reforming, and catalytic partial oxidation proceed according to the following reactions respectively:CH4+H2OCO+3H2  (1)CH4+CO22CO+2H2  (2) CH4+½O2→CO+2H2  (3)
While currently limited as an industrial process, catalytic partial oxidation (CPOX) has recently attracted much attention due to significant inherent advantages, such as the fact that heat is released during the process, in contrast to the endothermic steam and dry reforming processes.
Unfortunately, the heat production during a CPOX reaction can be a mixed blessing. In a CPOX reactor, there often occurs a substantial number of undesirable reactions, such as the non-selective oxidation of methane (e.g. to products other than CO and H2, for example, C, CO2 and H2O). Non-selective oxidation is much more exothermic than the desirable CPOX reaction (CH4+½O2→CO+2H2) and produces much more heat. This excess heat can have undesirable consequences. For example, excess heat at the front of the reaction zone can sinter the catalyst, causing a loss of surface area and, consequently, a loss of catalytic activity. In addition, if carbon is produced by the non-selective reactions, coking can be a problem. This loss of catalytic activity can, in turn, lead to an increase in the rate of unselective reaction, causing an even quicker deactivation of the catalyst, thus perpetuating a spiral of deactivation.
In catalytic partial oxidation, the hydrocarbon feedstock is mixed with an oxygen source, such as air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. When the feedstock comprises primarily methane, the approximately 2:1 H2:CO molar ratio achieved is generally more useful for downstream systems than the H2:CO ratio that is obtained from steam reforming. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis. An example of Fischer-Tropsch systhesis is disclosed in U.S. Pat. No. 6,365,544 to Herron et al., incorporated herein by reference.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. In many instances, the most effective catalyst compositions include precious and/or rare earth metals. These expensive catalysts needed for catalytic partial oxidation processes often place these processes generally outside the limits of economic justification. Additionally, some of the most selective catalysts, such as Rh, are not thermally stable enough to withstand the heat generated by the non-selective oxidation reactions that are inevitably present even using highly selective catalysts.
Typically, petrochemical applications of syngas require a specific molar ratio of hydrogen to carbon monoxide, such as, for example, 1:1 or 2:1. Current commercial processes for syngas generally yield much higher ratios. In addition, in some instances it is desirable to have a product stream comprising hydrogen with little or no CO. Therefore, separation technology, by-product credits, and production techniques that can adjust the hydrogen to carbon monoxide ratio and increase the overall feed hydrocarbon conversion and CO and H2 selectivity are important aspects of syngas production.
One such production technique which purports to increase the hydrogen ratio in the overall product stream is disclosed in MAIYA, P. S., et al., Maximizing H2 Production by Combined Partial Oxidation of CH4 and Water Gas Shift Reaction, 196 APPLIED CATALYSIS A: General 65-72 (2000) (Maiya et al.). MAIYA ET AL. discloses a two reactor system in which the first reactor is a methane partial oxidation reactor and the second reactor is a water gas shift (WGS) reactor that uses CO produced in the methane partial oxidation reactor as a feedstock (along with water) to produce hydrogen according to Equation 4:CO+H2O→CO2+H2  (4)This may seem a reasonable solution, but it requires a very large capital outlay in that two reactors, rather than one, are used. The present invention substantially alleviates the high cost of operating two reactors by its advantageous and surprising discovery that the reactions can take place in the same reactor.